The etiologies of dental caries and periodontal diseases are closely associated with dental plaque, the biofilm that develops from microbial colonization of the tooth surface. Colonization is initiated by a limited number of gram positive species, primarily different viridans streptococci and actinomyces which typically occur in vivo as members of a complex microbial community. The formation of this community most likely depends on an array of specific adhesive interactions which includes those detected by in vitro coaggregation between different bacteria. Interactions of this type are mediated by GalNAc or Gal sensitive adhesins on certain strains actinomyces or streptococci and complementary receptors on other strains of at least four viridans streptococcal species. The coaggregation receptors of these bacteria comprise a family of structurally related cell wall polysaccharides, each composed of a different phosphodiester linked hexa- or heptasaccharide repeating unit. Lectin recognition of these molecules depends on the presence of a host- like disaccharide motif, either Gal-beta-1 -> 3GalNAc or GalNAc-beta-1 -> 3Gal, in each repeating unit. In contrast, the antigenic cross reactivity of different receptor polysaccharides is more closely correlated with the non host-like features of these molecules. Current studies are directed toward identification, characterization and comparison of the gene clusters for three different but structurally related receptor polysaccharides. Various experimental approaches are being pursued including characterization of transposon insertion mutants that are negative of receptor polysaccharide production and genetic complementation to clone and characterize genes for polysaccharide biosynthesis in viridans streptococci. Information gained from these studies may well contribute to the development of novel molecular approaches to assess the recognition role of streptococcal receptor polysaccharides in microbial colonization and biofilm formation.